CP7’s decarbonisation drive puts spotlight on power quality

Network Rail’s Control Period 7 (CP7) puts the focus on decarbonisation, with heating building stock set to become a key target area for carbon reduction. However, potential power quality issues mean that it’s not just a matter of switching out an old boiler for a heat pump.

David Bean is business development group manager at Mitsubishi Electric – Automation Systems UK

CP7, which is due to run from July 2024 to June 2029, has decarbonisation as one of its central themes, with Network Rail pledging to further reduce carbon emissions across the network by 20.5%.

The decarbonisation of heating systems in stations, depots and office buildings offers an enormous opportunity for the rail industry. Ageing infrastructure that can be upgraded with modern, more efficient heating systems can help reduce the environmental impact of the rail network and enable Network Rail to make big strides towards its targets.

The replacement of old gas boilers with heat pumps is one way of making a carbon saving, but the swapping out of old technology for new presents challenges, particularly for the electricity network.

Heat pump technology uses variable speed drives (VSDs) to ensure that the pump is not running at full power when not necessary, thus contributing to its energy saving capabilities.

However, VSDs, like computers and LED lights have non-linear load characteristics that, in layman’s terms, draw pulses of power rather than a steady flow. When multiple heat pumps are grouped together, typically in a commercial or industrial building, these pulses can introduce harmonic distortion into the power system.

Harmonics distort the usual smooth sinusoidal waveform and lead to what are known as “power quality issues”. These can have a detrimental effect on wider electrical equipment and IT systems, ultimately affecting performance and shortening the expected productive life of the equipment.

These issues can take many forms, from simple overheating through to reduced system efficiency, or a reduction in the overall power flow to a building or facility. It is not unheard of for nearby homes and other business premises to be affected, with lights unexpectedly dimming as the quality of electricity available in the local network is reduced.

Power quality issues are not confined to the rail industry. Mitsubishi Electric has been working on numerous power quality related projects to ensure that the grouping of multiple heat pumps in other commercial or industrial spaces such as schools or hospitals have no adverse effect on the smooth functioning of the facility, such as mission critical IT systems.

Energy power suppliers responsible for the local power supply are also increasingly focused on this issue, with many now demanding a G5/5 certificate for a site or project in order to demonstrate that harmonic mitigation countermeasures are in place before signing off on an installation.

The G5/5, introduced in 2020 by the Energy Networks Association, helps guide any assessment of potential harmonic distortion in a local network and sets limits for the connection of non-linear equipment. A G5/5 is essential in order to gain permission from a District Network Operator (DNO) to connect new or additional harmonic producing loads, such as multiple heat pumps, to the network.

As a result, end users, specifiers and HVAC consultants have had to think much more carefully about harmonic assessment during the design and specification phase of a project. Increasing the volume of equipment and drawing greater power means that harmonic survey and design is essential for each individual site and project.

With CP7 still very much in its infancy, knowledge levels within the rail industry about harmonic distortion and power quality issues are understandably low at the present time. However, with designs for new stations and depots well underway, this issue is sure to become more of a focus.

My advice is two-fold. Firstly, place potential power quality issues on the table early and get a power quality site survey completed that looks at all the challenges, from voltage, through to current, harmonic distortion, frequency, flicker, voltage sag and motor and transformer inrush.

Secondly, if power quality issues are found, or have the potential to become apparent with an installation, then it is important to put corrective action in place early. Harmonic filters that can block harmonic currents at source are usually a suitable countermeasure.

CP7’s focus on decarbonisation is welcome, but wider stakeholders in the rail network, specifically the DNOs, need to be consulted and kept onside if the opportunities are to be realised.